In a two-cycle internal combustion engine, occurrence of detonation (abnormal combustion) is inhibited by delaying an ignition timing from an ignition timing in a normal state when detonation occurs, and the ignition timing is returned to the timing in the normal state when the detonation does not occur any longer.
In the two-cycle internal combustion engine, air-fuel mixture in a cylinder is ignited by an ignition plug, then a flame propagates in a combustion chamber. In a process where the flame propagates in the combustion chamber, burned gas compresses unburned gas in the surroundings to excessively increase pressure of the unburned gas. When a temperature of the unburned gas is increased by the pressure increase and radiant heat from the flame, self-ignition of the unburned gas is caused and thus explosive abnormal combustion in the cylinder occurs. Such a phenomenon is referred to as detonation or knocking and causes damage to the engine. Because this phenomenon occurs when the amount of advance of an ignition timing is large or when an air-fuel ratio of the air-fuel mixture goes lean and a cooling effect of a piston by latent heat of evaporation of fuel is thus weak, the phenomenon can be inhibited by delaying the ignition timing or controlling the air-fuel ratio to a rich side, and generally, an ignition timing in a normal state is set to a delay side in view of variation in the air-fuel ratio. In the two-cycle internal combustion engine, however, a maximum output is obtained in a state immediately before the detonation occurs or a state where the detonation occurs to some extent, and thus high performance of the engine cannot be achieved by setting the ignition timing to the delayed side.
Thus, in a high-performance two-cycle internal combustion engine used in a motorcycle or a snowmobile, an ignition timing is set so that an engine is operated in a state immediately before detonation occurs or a state where the detonation occurs to some extent, and detonation inhibition control is performed to protect the engine. In the detonation inhibition control, control is performed so that when detonation is detected, an ignition timing is delayed to reduce indicated means effective pressure (IMEP) and thus inhibit occurrence of the detonation, and when the occurrence of the detonation stops, the ignition timing is advanced. Such control inhibits the occurrence of the detonation and allows high performance of the engine to be achieved without sacrificing an output of the engine. An ignition control method for controlling to delay an ignition timing at the occurrence of detonation in order to inhibit the occurrence of the detonation is disclosed in, for example, Japanese Patent Application Laid Open Publication No. 6-108955.
In the two-cycle internal combustion engine, an exhaust pipe is designed to have a shape such that a pressure wave of exhaust gas is reflected in the exhaust pipe, and unburned gas blowing through in a scavenging stroke is pushed back toward a cylinder by the reflected pressure wave, thereby increasing a charging efficiency into the cylinder. The exhaust gas pressure wave propagates in the exhaust pipe by an acoustic velocity, and thus the effect of increasing the charging efficiency into the cylinder by the reflected exhaust gas pressure wave (hereinafter referred to as an exhaust pipe effect) is influenced by an acoustic velocity change resulting from an exhaust pipe temperature change.
The amount of fuel supplied to the two-cycle internal combustion engine is decided assuming that an exhaust pipe temperature is within an assumed range and that the exhaust pipe effect serves effectively to increase the charging efficiency. Thus, if the exhaust pipe temperature increases beyond the assumed range, the large amount of unburned gas blowing through remains in the exhaust pipe, and burns in the exhaust pipe. When the unburned gas burns in the exhaust pipe to increase the exhaust pipe temperature, the exhaust pipe effect is lost to increase the unburned gas remaining in the exhaust pipe, thereby further increasing the exhaust pipe temperature. In the two-cycle internal combustion engine, if the unburned gas burns in the exhaust pipe to increase the exhaust pipe temperature beyond the assumed temperature range, positive feedback may be applied to the increase in the exhaust pipe temperature, thereby causing the exhaust pipe temperature to abnormally increase.
Generally in a two-cycle internal combustion engine, an exhaust gas temperature increases when an ignition timing is delayed. Thus, in a two-cycle internal combustion engine that performs detonation inhibition control for delaying an ignition timing at the occurrence of detonation in order to inhibit detonation, the detonation inhibition control is continuously performed or repeatedly performed at short time intervals to overheat an exhaust pipe, which may cause an accident such as a fire or reduce the life of the exhaust pipe.